1. Field of the Invention
The proposed Broadband-Integrated Services Digital Network (B-ISDN) requires switching of data using the Asynchronous Transfer Mode (ATM) multiplex structure, with all services placed in cells of 53 octets including routing information.
2. Description of Related Art
The existing network employs Synchronous Transfer Mode (STM) to carry individual octets along predetermined routes, controlled by stores in the switches.
For these reasons there are three possible strategies for the introduction of ATM based B-ISDN alongside the current STM network:
1) Overlay Network.
                The addition of an ATM network alongside the existing STM network. B-ISDN services are carried on the overlay, and existing services remain on the STM.        This solution requires major capital investment to introduce ATM at a national level, and there is no guarantee that the service take-up will be sufficient to cover this cost within a reasonable time. This also limits the flexibility of management of the network.2) Emulation Over ATM.        ATM can provide a “circuit emulation” of an STM circuit, and this is often seen as the way to support STM services on an ATM network.        It has been assumed that the existing STM services (e.g. voice) can eventually be carried over the ATM network by adapting the service into the ATM cells. This produces delays for packetisation, and also requires the STM service to be resilient against cell loss, which many services (notably H.261 video) are not yet able to cope with.        Therefore, while this may be possible when ATM is a more mature environment, the services are not yet sufficiently developed to use ATM.3) Dual Mode Network.        An arrangement which provides the ability to switch both ATM traffic and STM traffic in the same switch fabric. This avoids the problems above, and has other benefits as well. Accordingly, it is required to make it possible.        